The instant invention relates generally to hydrogen storage alloys and more specifically to CaNi5 electrochemical hydrogen storage alloys. Most specifically, the instant invention relates to modified CaNi5 alloys which are electrochemically-stabilized for use in metal hydride batteries.
Rechargeable cells that use a nickel hydroxide positive electrode and a metal hydride forming hydrogen storage negative electrode (xe2x80x9cmetal hydride cellsxe2x80x9d) are known in art. The first hydrogen storage alloys to be investigated as battery electrode materials were TiNi and LaNi5. Many years were spent in studying these simple binary intermetallics because they were known to have the proper hydrogen bond strength for use in electrochemical applications. Despite extensive efforts, however, researchers found these intermetallics to be extremely unstable and of marginal electrochemical value due to a variety of deleterious effects such as slow discharge and poor cycle life brought about by oxidation, corrosion, poor kinetics, and poor catalysis. These simple alloys for battery applications reflect the traditional bias of battery developers toward the use of single element couples of crystalline materials such as NiCd, NaS, LiMS, ZnBr, NiFe, NiZn, and Pb-acid. In order to improve the electrochemical properties of the binary intermetallics while maintaining the hydrogen storage efficiency, early workers began modifying TiNi and LaNi5 based alloys.
In U.S. Pat. No. 4,623,597 (xe2x80x9cthe ""597 patentxe2x80x9d), the contents of which are incorporated by reference, one of the present inventors, Ovshinsky, described disordered multicomponent materials for use as negative electrodes in electrochemical cells for the first time. In this patent, Ovshinsky describes how disordered materials can be tailor made to greatly increase hydrogen storage and reversibility characteristics. Such disordered materials are amorphous, microcrystalline, intermediate range order, and/or polycrystalline (lacking long range compositional order) wherein the polycrystaline material includes topological, compositional, translational, and positional modification and disorder. The framework of active materials of these disordered materials consist of a host matrix of one or more elements and modifiers incorporated into this host matrix. The modifiers enhance the disorder of the resulting materials and thus create a greater number and spectrum of catalytically active sites and hydrogen storage sites.
The disordered electrode materials of the ""597 patent were formed from lightweight, low cost elements by any number of techniques, which assured formation of primarily non-equilibrium metastable phases resulting in the high energy and power densities and low cost. The resulting low cost, high energy density disordered material allowed the batteries to be utilized most advantageously as secondary batteries, but also as primary batteries.
Tailoring of the local structural and chemical order of the materials of the ""597 patent was of great importance to achieve the desired characteristics. The improved characteristics of the anodes of the ""597 patent were accomplished by manipulating the local chemical order and hence the local structural order by the incorporation of selected modifier elements into a host matrix to create a desired disordered material. The disordered material had the desired electronic configurations which resulted in a large number of active sites. The nature and number of storage sites was designed independently from the catalytically active sites.
Multiorbital modifiers, for example transition elements, provided a greatly increased number of storage sites due to various bonding configurations available, thus resulting in an increase in energy density. The technique of modification especially provides non-equilibrium materials having varying degrees of disorder provided unique bonding configurations, orbital overlap and hence a spectrum of bonding sites. Due to the different degrees of orbital overlap and the disordered structure, an insignificant amount of structural rearrangement occurs during charge/discharge cycles or rest periods therebetween resulting in long cycle and shelf life.
The improved battery of the ""597 patent included electrode materials having tailor-made local chemical environments which were designed to yield high electrochemical charging and discharging efficiency and high electrical charge output. The manipulation of the local chemical environment of the materials was made possible by utilization of a host matrix which could, in accordance with the ""597 patent, be chemically modified with other elements to create a greatly increased density of catalytically active sites for hydrogen dissociation and also of hydrogen storage sites.
The disordered materials of the ""597 patent were designed to have unusual electronic configurations, which resulted from the varying 3-dimensional interactions of constituent atoms and their various orbitals. The disorder came from compositional, positional and translational relationships of atoms. Selected elements were utilized to further modify the disorder by their interaction with these orbitals so as to create the desired local chemical environments.
The internal topology that was generated by these configurations also allowed for selective diffusion of atoms and ions. The invention that was described in the ""597 patent made these materials ideal for the specified use since one could independently control the type and number of catalytically active and storage sites. All of the aforementioned properties made not only an important quantitative difference, but qualitatively changed the materials so that unique new materials ensued.
The disorder described in the ""597 patent can be of an atomic nature in the form of compositional or configurational disorder provided throughout the bulk of the material or in numerous regions of the material. The disorder also can be introduced into the host matrix by creating microscopic phases within the material which mimic the compositional or configurational disorder at the atomic level by virtue of the relationship of one phase to another. For example, disordered materials can be created by introducing microscopic regions of a different kind or kinds of crystalline phases, or by introducing regions of an amorphous phase or phases, or by introducing regions of an amorphous phase or phases in addition to regions of a crystalline phase or phases. The interfaces between these various phases can provide surfaces which are rich in local chemical environments which provide numerous desirable sites for electrochemical hydrogen storage.
These same principles can be applied within a single structural phase. For example, compositional disorder is introduced into the material which can radically alter the material in a planned manner to achieve important improved and unique results, using the Ovshinsky principles of disorder on an atomic or microscopic scale.
One advantage of the disordered materials of the ""597 patent were their resistance to poisoning. Another advantage was their ability to be modified in a substantially continuous range of varying percentages of modifier elements. This ability allows the host matrix to be manipulated by modifiers to tailor-make or engineer hydrogen storage materials with all the desirable characteristics, i.e., high charging/discharging efficiency, high degree of reversibility, high electrical efficiency, long cycle life, high density energy storage, no poisoning and minimal structural change. These same attributes can be achieved for the alloys of the subject patent application.
Throughout the development of hydrogen storage alloys for electrochemical use, certain development principles have applied. Among these are need for the alloy material to 1) be stabile in the corrosive alkaline battery environment, 2) be formed from low cost materials, 3) have the highest possible specific energy density. In order for the alloys to have the highest possible specific energy density, the alloys must be formed from light weight materials and/or store large amounts of hydrogen. Calcium alloys have the ability to store hydrogen and are light weight. Also, calcium is relatively low in cost compared to other hydrogen storage elements. However, as calcium is notorious for it""s reactivity in an alkaline environment, to date no calcium alloys have been found which are useful in the corrosive alkaline environment of electrochemical cells. One specific calcium alloy which has been used in the past is CaNi5, see Japanese Published Application No. 53-019,129. Attempts to modify CaNi5 for electrochemical applications were made in the 1970s and 1980s by researchers in Japan. See for instance Japanese Published Applications Numbers: 54-011,095; 55-154,301; 56-169,746; 58-096,842; 58-096,843; 60-172,165; 60-215,724; 61-019,059; 61-019,061; 61-019,062; and 61-168,869. None of these modified alloys proved to be stable materials and therefore are not commercially viable CaNi5 alloys.
Therefore, there is still an urgent need in the art for a low cost, light weight, electrochemically stable alloy with high energy density for use as the negative electrode of a metal hydride electrochemical cell.
In the broadest sense, the object of the instant invention is a material having at least one crystalline phase defined by a crystal unit cell formed by at least a first element which is substantially inert to degradation from the intended environment of the material and a second element which is subject to degradation from said environment. The unit cell of the material is formed with the first element occupying lattice sites of each unit cell to form a channel within which the second element occupies interior lattice sites. The second element is subject to degradation from unsealed ends of said channel and the improvement is the addition of a third element adapted to atomically engineer the local structural environment of the unit cell such that at least some atoms of the first element occupy some of the sites within the interior of the channel which are normally occupied by atoms of the second element. This seals the channel and prevents the environmental degradation of the second element. The material can also have a fourth element adapted to further atomically engineer the local structural environment of the unit cell. The fourth element has a stronger bond to the second element than does the first element. The fourth element displaces at least some of the first element in the unit cell, thereby holding the second element more strongly within the channel.
More specifically, the object of the instant invention is an electrochemically stabilized Caxe2x80x94Ni hydrogen storage alloy material for use as the active negative electrode material of an alkaline electrochemical cell. The alloy material includes at least one modifier element which stabilizes the alloy material from electrochemical degradation. during electrochemical cycling in an alkaline electrochemical cell, by protecting calcium within the material and preventing dissolution of calcium into the alkaline electrolyte.
In one embodiment, the electrochemically stabilized Caxe2x80x94Ni hydrogen storage alloy has the formula (Ca1xe2x88x92xxe2x88x92yMxNi2y)Ni5, where M is at least one element selected from the group consisting of misch metal, rare earth metals, zirconium or mixtures of Zr with Ti or V, x ranges between about 0.02 and 0.2, and y ranges between about 0.02 and 0.4. M is preferably zirconium.
In another embodiment, the alloy has the formula (Ca1xe2x88x92xxe2x88x92yMxNi2y)Ni5xe2x88x92zQz, where M is at least one element selected from the group consisting of misch metal, rare earth metals, zirconium and mixtures of Zr with Ti or V, Q is at least one element selected form the group consisting of Si, Al, Ge, Sn, In, Cu, Zn, Co, and mixtures thereof, x ranges between about 0.02 and 0.2, y ranges between about 0.02 and 0.4, and z ranges from about 0.05 to about 1.00. M is preferably zirconium and Q is preferably silicon.